1. Field of the Invention
The present invention relates to an adaptive equalizer applicable to a modem that carries out communication according to a frequency multiplexing system and a communication apparatus using the adaptive equalizer.
2. Description of the Related Art
As a technology for implementing high-speed data communication using a metallic cable, an ADSL (Asymmetric Digital Subscriber Line) is being developed. In an ADSL apparatus, transmission data is assigned to a plurality of frequencies, a few bits each, subjected to IFFT (inverse fast Fourier transform) and sent out to a transmission path. In the explanations hereafter, this system will be referred to as a “frequency multiplexing system”. The transmission data is divided into units of IFFT processing as one symbol, and a signal with a guard band to guard against interference between symbols added is sent out as one symbol. For example, in the case of G. Lite, which is one of the ADSL standards, 16 samples of guard band are added to 256 samples of data over the forward link. In the actual line, interference between symbols is expected to be longer than a guard band, and therefore the receiver is provided with an adaptive equalizer to reduce interference between symbols. The Unexamined Japanese Patent Publication NO.HEI 7-516790 discloses an adaptive equalizer training circuit that adapts a tap coefficient of an FIR type filter used as an adaptive equalizer quickly and stably.
FIG. 1 is a functional block diagram of such an adaptive equalizer training circuit. During training of the adaptive equalizer, in transmitter 200, PRBS generator 201 generates a PRBS (pseudo-random sequence) signal, encoder 202 encodes the PRBS signal and generates frequency area signal X (D), IFFT section 203 converts X(D) to time area signal x(t) and sends out this to transmission path 204. The present specification generally expresses a frequency area signal in an alphabetic uppercase letter with a suffix (D) and a time area signal in an alphabetic lowercase letter with a suffix (t).
Here, suppose discrete time impulse response of transmission path 204 is h(t) and a mixed noise signal is n(t). Then, signal y(t) received by the receiver is expressed in Expression (1).y(t)=x(t)*h(t)+n(t)  (1)
where, “*” denotes a convolutional calculation.
In training circuit 205 on the receiver side, the adaptive equalizer asymptotically updates the value of target impulse response BW(D) using a frequency area vector of an impulse response that can be equalized to a fixed impulse response length of v taps or less as a target. For this purpose, PRBS generator 206 generates a PRBS signal in synchronization with the transmitter side, encoder 207 converts the PRBS signal to signal X(D) and target impulse response updating section 208 calculates update target impulse response Bu (D).
Target impulse response updating section 208 subjects reception signal y(t) to FFT (fast Fourier transform), obtains frequency area reception signal Y(D) and calculates target impulse response BU(D) indicating a difference between the reception signal after passing the adaptive equalizer and the signal to be received originally according to expression (2).BU(D)=Y(D)×WW(D)/X(D)  (2)
where, WW(D) is a tap coefficient of an FIR type filter.
Target impulse response time window section 209 applies IFFT processing to target impulse response BU(D) and converts this to a time area signal, extracts tap coefficient bW(t) through a time window of a fixed length of v taps, applies FFT processing to tap coefficient bW(t) and converts this to frequency area target impulse response signal BW(D) of a fixed length of v taps.
Tap coefficient updating section 210 calculates an error from Expression (3) using X(D), BW(D), Y(D) and WW(D) and obtains tap coefficient WU(D) corresponding to a minimum of error E(D) using an LMS method (least-square method).E(D)=X(D)×BW(D)−Y(D)×WW(D)  (3)
WU(D) is updated as:WU(D)=WW(D)+2μ×E(D)×X*(D)  (4)
where, μ denotes a step size of LMS and X* denotes a complex conjugate of X.
Tap coefficient time window section 211 extracts the updated WU(D) by a length corresponding to the number of taps calculated when the actual signal is received as the value of WW(D).
In the training of the adaptive equalizer above, the transmitter and receiver generate PRBS repeatedly in synchronization with each other, training circuit 205 repeats the above procedure until error E(D) converges on a threshold or below and tap coefficient WW(D) finally obtained is used as the tap coefficient of the adaptive equalizer. This can equalize the impulse response to a sufficiently short length even on a poor transmission path and eliminate interference between symbols and interference between channels.
However, in the adaptive equalizer above, target impulse response updating section 208 performs FFT processing once, target impulse response time window section 209 performs IFFT processing once, tap coefficient updating section 210 performs FFT processing twice and tap coefficient time window section 211 performs IFFT processing once, and these calculations are repeated for every symbol. Thus, the adaptive equalizer above has a problem that the amount of calculation becomes enormous, and since calculations are made by reciprocating between the time area and frequency area, the adaptive equalizer above also has a problem that the filter factor includes errors, which reduces reliability.